1. Field of the Invention
The present invention relates to an improvement of a semiconductor laser and method of making the same.
2. Description of the Prior Art
In recent years, there is a large demand for semiconductor lasers which can lase a high power output in the fundamental transverse mode, for instance for use in writing on an optical disk file or the like. In order to stabilize the transverse mode lasing, it is a very effective measure to provide a groove on the substrate.
As one prior art example of such a semiconductor laser, a CSP (channeled substrate planar) laser is shown in FIG. 1. The CSP laser shown in FIG. 1 comprises an n-GaAs substrate 1, an n-Ga.sub.1-x Al.sub.x As first clad layer 2, a non-doped Ga.sub.1-y Al.sub.y As active layer 3, p-Ga.sub.1-x Al.sub.x As second clad layer 4 and n-GaAs cap layer 5. A p-side metal electrode 6 and an n-side electrode 7 are respectively provided on the upper face and bottom face of the wafer. And an ohmic electrode 8 is formed by diffusing Zn to extend into the second clad layer 4 by a thermal diffusion process.
In this semiconductor laser, the substrate 1 has a channel 11 which has upwardly opening side walls 10, 10, so that the first clad layer 2 has a thicker part on the part of the channel 11 and thinner part on both sides thereof. By such a configuration, light which is lased in the active layer 3 and leading towards the substrate 1 is absorbed with different degrees, that is, much is absorbed in the thinner parts. Thereby, the transverse mode oscillation is well confined in the active layer 3 at the part which is just above the groove, thereby realizing fundamental transverse mode lasing.
Generally speaking, absorption of lased light in the substrate should be made as small as possible in order to realize a high output lasing in the semiconductor laser to thereby increase the external differential quantum efficiency. In the CSP laser of the prior art shown in FIG. 1, since it is manufactured by using a forward mesa-substrate, the shape of the groove 11 becomes an upwardly open shape, and this groove is not suitable for attaining a high output of lasing. This is because if a very deep groove is intended to be made in order to confine a sufficient amount of lased light, the inclined side walls 10, 10 of the groove 11 (a-b and c-d) become long. Accordingly, light absorption at these broad walls becomes large, thereby making achievement of a high external differential quantum efficiency difficult.